The present invention relates to laminates, and more particularly laminates having good formability in processes such as thermoforming and vacuum skin packaging.
It is common practice to package articles such as food products in thermoplastic films or laminates to protect the product to be packaged from abuse and exterior contamination, and to provide a convenient and durable package for transportation and ultimate sale to an end user.
Thermoforming and other similar techniques are well known in the art for packaging food products. Suitable thermoforming methods, for example, include a vacuum forming or plug-assist vacuum forming method. In a vacuum forming method, the first web is heated, for example by a contact heater, and a vacuum is applied beneath the web causing the web to be pushed by atmospheric pressure down into a preformed mold. In a plug-assist vacuum forming method, after the first or forming web has been heated and sealed across a mold cavity, a plug shape similar to the mold shape impinges on the forming web and, upon the application of vacuum, the forming web transfers to the mold surface.
After the forming web is in place, a product is placed, such as by manual loading, on the forming web and a second substantially non-forming web is disposed over the product. The package is vacuumized and fusion sealed with a sealing device such as a heated jaw. The first or forming web encloses a substantial portion, generally more than half, of the product to be packaged.
Thermoforming is a popular method of packaging meat products such as smoked and processed meats. In packaging such products, it is desirable to provide a clear package with good optical properties such as clarity and gloss in order to enhance package appearance for the consumer.
Color retention of the product, particularly in the case of meat products, and extended shelf life are also advantageous features. Finally, it is desirable to provide a packaging material which, after the thermoforming operation, will maintain a tight package appearance commonly known in the art as good memory characteristics.
A description of typical thermoforming techniques currently available in the art appears in Modern Plastics Encyclopedia, 1984-1985, at pages 329-336.
Vacuum skin packaging (VSP) is another process well known in the art for using a thermoplastic packaging material to enclose a product such as meat. Various apparatus and processes are described in U.S. Pat. Nos. 3,835,618, 3,950,919, and reissue 30,009, all issued to Perdue. Cross linked polyolefin materials useful in VSP applications, and providing readily peelable seals, are described in U.S. Pat. No. 4,382,513 issued to Schirmer et al. The vacuum skin packaging process is in one sense a type of thermoforming process in which an article to be packaged serves as the mold for the forming web. An article may be placed on a support member, a card, either rigid or semi-rigid, or other bottom web, and the supported article is then passed to a chamber where a top web is drawn upward against a heated dome and the softened top web is then draped over the article. The movement of the web is controlled by vacuum and or air pressure, and in a vacuum skin packaging arrangement, the interior of the container is vacuumized before final welding of the top web to the support web. Formable material may be used as both top and bottom webbing in conjunction with an intermediate support for products held on either side of the support as shown for example in U.S. Pat. No. 3,966,045.
One common requirement of the forming web in thermoforming processes, and a skin packaging material in VSP processes, is the requirement of some degree of formability of the material. A common and recurrent problem in such operations is the occurrence of wrinkles, line draw and other irregularities in the final packaged product. In VSP applications, wrinkles can occur especially at lower temperatures, and efforts have been made to reduce wrinkles in the final package by means of higher forming temperatures.
Thus, in a VSP application, the dome temperature i.e. the temperature of the top of the chamber to which a VSP film is drawn and heated for softening, is raised to for example 190.degree. to 270.degree. C. range.
Some improvement in providing a wrinkle-free package may be obtained by this solution, but it is not without other complications. For example, it is obviously more expensive to run a piece of equipment at the elevated temperature needed to reduce wrinkles. The package produced at the elevated temperatures may not be as aesthetically pleasing as one produced at lower temperatures, for example in the case of hard chilled meat, where the preferred rigid tabular appearance of the meat cut may be softened or distorted by exposure to the elevated temperatures. In cases where the elevated temperatures are required by the properties of the constituent resins of the packaging material, it may be impractical or impossible to operate at significantly lower temperatures and still produce a suitable package.
It is therefore an object of the present invention, to provide a film useful in forming processes such as vacuum skin packaging or thermoforming, where relatively low temperatures may be utilized in heating the material.
It is an additional object of the present invention to provide a film which will result in no line draw in thermoforming or reduced wrinkles in a VSP operation. Line draw is used herein to describe an undesirable characteristic of a thermoplastic crystalline material which has been produced with either substantially no orientation, or orientation principally in one direction only, usually the machine direction. Upon subsequent thermoforming of the film, the film is stretched as it is shaped to the thermoformed mold configuration. In the unoriented direction, the material will elongate at the yield point, resulting in elongated tapered streaks aligned in the oriented direction.
It is an additional object of the present invention to provide a packaging film incorporating an oxygen barrier material which may be formed at relative low temperatures.